Throughout the world, considerable oil reserves may be found locked in the form of tar/oil sand, also known as bitumen sand. Bitumen, which is a viscous hydrocarbon, is trapped between the grains of sand, clay, and water. Because the recovery of bitumen from the sand may provide an increasingly valuable commercial energy source, processes for extracting and refining bitumen have long been investigated.
One method for recovering tar sand is by mining. In these operations, surface or shallow oil sands are open pit mined. The cost of mining increases with the depth of burial of the formation. At some point, the amount of overburden and the cost of its removal becomes too great. These deeper deposits have recently begun to be exploited by drilling wells through the overburden. In some cases, the bitumen behaves as a fluid under reservoir conditions, and may flow into the well for production by conventional means. However, in other cases, the bitumen is either too viscous or is too solidified, and may not flow. To recover these deposits, steam or other heat sources may be introduced into the tar sand formation to liquefy the bitumen. Recently, a technique of drilling closely spaced horizontal wells that allow a controlled passage of steam therebetween has become popular. After months of steaming, the molten tar flows into collection wells for recovery. So-called Steam Assisted Gravity Drainage is one such technique.
In Alberta, the tar sands underlie a wide expanse of undeveloped and environmentally sensitive areas in the north of the province. Drilling wells inevitably creates large amounts of overburden and tar sand cuttings. Currently, tarred cuttings must be hauled to either existing mining operations or permitted disposal sites. Therefore, processes that separate tar from sands at the drill site and allow delivery of sands clean enough for on-site disposal may reduce the cost of drilling.
Similar problems may occur when attempting to remove tar from drilled cuttings as those encountered when trying to recover tar from mined sand. However, when removing tar from drilled cuttings, surfactants, substances present in drilling fluid, and substances otherwise used to facilitate tar removed during the drilling process may contaminate the drilled cuttings. Such substances may cause environmental concerns if not removed from the drilled cuttings prior to disposal.
Currently, extraction of the bitumen from oil sand and drilled cuttings may be accomplished though a number of different processes. One process involves mixing the oil sand with hot water, an example of which is disclosed in U.S. Pat. No. 5,626,741, hereby incorporated by reference herein. In the hot water extraction process, oil sands are first conditioned in large conditioning drums or tumblers with the addition of NaOH and water at a temperature of about 85° C. The tumblers provide means for steam injection and physical action to mix the resultant slurry vigorously, causing the bitumen to be separated from the oil sands, and then aerated to form bitumen froth.
The slurry from the tumblers is then screened to separate out the larger debris and passed to a separating cell where settling time is provided to allow the slurry to separate. As the slurry settles, the bitumen froth rises to the surface and the sand particles and sediments fall to the bottom. A middle viscous sludge layer, termed middlings, contains dispersed clay particles and some trapped bitumen that is not able to rise due to the viscosity of the sludge. Once the slurry has settled, the froth is skimmed off for froth treatment and the sediment layer is passed to a tailings pond. The middlings are often fed to a secondary flotation state for further bitumen froth recovery.
Bitumen froth contains bitumen, solids, and trapped water. The solids that are present in the froth are in the form of clays, silt, and sand. From the separating cell, the froth is passed to a defrothing or deaerating vessel where the froth is heated and broken to remove the air. Typically, naphtha is then added to solvate the bitumen to reduce the density of the bitumen and to facilitate separation of the bitumen from the water by means of a subsequent centrifugation treatment. The centrifuge treatment typically involves a gross centrifuge separation followed by a series of high-speed centrifuge separations. The water and solids released during the centrifuge treatment are passed to the tailings pond, while recovered bitumen may then be transferred for further processing.
When bitumen is treated using the conventional naphtha dilution and centrifugation extraction process, considerable problems may be encountered. First, the naphtha-diluted bitumen product may contain up to 5 wt % water and solids. Second, the naphtha dissolves the bitumen as well as the unwanted and dirty asphaltenes contained in the bitumen froth. The contamination of bitumen oil may result in inefficient end product production, specifically, when hydrocracking is used. Hydrocracking is a process which uses hydrogen gas and a catalyst to separate a reagent into various products. Hydrocracking may produce, among other end products, naphtha and distillates. Because hydrocracking requires a homogeneous feed, which is low in solids and water, the naphtha diluted bitumen product cannot be fed directly to the hydrocracker. In order to use the naphtha diluted bitumen product, it must first be coked to drive off the naphtha solvent and drop out the asphaltenes and solids. Unfortunately, this coker upgrading represents a substantial capital outlay and results in a loss of 10-15% of the bitumen initially available for hydrocracking.
Additional methods of further removing bitumen from oil sand have also been proposed, including a method for cleaning post-primary bitumen froth (i.e. bitumen froth collected after initial skimming) containing bitumen, water, and solids, which is disclosed in U.S. Pat. No. 5,290,433, hereby incorporated by reference herein. This method includes introducing a bitumen-containing solution into a chamber through a tube carrying one or more pairs of opposed throw propellers. The propellers shear the froth, causing the froth to exit the tube in different directions, thereby separating the solids from the aerated bitumen which rises to the top, forming a new froth. The newly formed bitumen-containing froth may then be collected, while the middlings are withdrawn from the chamber and recycled to join the feed. While this process of removing bitumen is useful in collecting bitumen from post-primary bitumen froth, its utility is limited in that the middlings are simply recycled through the same process.
Because of the limitations of single step systems, as those disclosed above, larger systems have been developed to more efficiently remove bitumen from oil sand. One such system is disclosed in U.S. Pat. No. 5,795,444, which is hereby incorporated by reference herein. In this process, the oil sand is stirred to form a slurry with hot water and steam. The injection of hot water and steam may cause bitumen oils, sand, and water, to segregate into layers in a flotation vessel. The flotation vessel is then skimmed to remove the bitumen oil from the sand and water, while the remaining slurry is transferred to a hydrocyclone. The hydrocyclone further separates bitumen oil from the slurry, diverting the hydrocyclone overflow to a thickening vessel. The remaining bitumen oil then floats to the surface of the thickening vessel, while any remaining water and sand are transferred to a sand washer, whereby the process repeats.
While this system provides multiple means for separating bitumen from sand, its effectiveness is limited by the single flotation cell skimmer. Additionally, the system does not provide a means for recycling water throughout the process. Thus, the advantages of the system are restricted by the constant need for water, as well as the inefficiency of a system that only extracts bitumen from a single source, namely the flotation cell skimming.
Such processes as those mentioned above have not facilitated the efficient extraction of bitumen oil from oil sands. The aforementioned processes either haven't been adopted by the industry due to the fact that they substantially increase the cost of bitumen extraction, or have been adopted but result in high levels of hazardous waste product. Accordingly, there exists a need for a process that increases the production of bitumen oil from oil sand, while decreasing levels of hazardous waste and producing substantially cleaner sands.